In a cellular radio communications system, mobile radio stations communicate with one or more base stations over an air or radio interface. Communication links between mobile subscribers and base stations should be established quickly, and handover of a mobile connection between base stations should be performed rapidly, without errors, and without inadvertently dropping the call or losing a communication link. These objectives are accommodated by the mobile station scanning signals broadcast from nearby base stations. Scanning is analogous to “listening” to neighboring base stations to determine which base station to “camp on” if the mobile is not involved in an active connection and to determine whether handover of an active connection with the mobile station to another base station should occur.
Scanning can be both time consuming and power consuming. A mobile station must repeatedly scan many base stations both while it is in an idle state and when it is involved in an active connection. For each scan of a broadcast from a base station in an associated cell, the mobile must tune to the frequency of the broadcast, determine a signal strength and/or signal quality of the broadcast signal, acquire frequency and time synchronization with the broadcast, and decode the broadcast information which often includes various system and cell-specific information. From the decoded broadcast information, the mobile station can determine whether it is permitted to obtain any kind of service from this cell, i.e., the cell is not a “forbidden” cell, and if so, whether the cell offers the kind of service the mobile station wants. Some example services include: a cell may only provide limited resources on the uplink and/or downlink for acquiring a video service; a cell may not support the mobile station's power profile or multi-broadcast service zone; or a cell may not provide certain handovers. These scanning related operations consume the limited battery resource that powers the mobile station.
Most broadcast signals are transmitted as a series of frames, each frame having a known signal called a preamble used by mobile stations for synchronization. Each frame also has other basic synchronization information about the cell. However, service information required to obtain the service from the cell is broadcasted less frequently. In some situations, the time period between service information transmissions may be very long. As a result, the mobile station may have a significant time period after it has synchronized to the base station broadcast before it receives that service-related information. This makes the scanning process both time and power inefficient.